In recent times, the development of a mobile communication industry represented by mobile phones is making rapid progress. With this rapid progress, various services by means of mobile communications are also diversified. Particularly, future needs for mobile communications necessarily move to multimedia communications for integrating and communicating various kinds of data including not only voices but also texts, images and the like, and the combination of those needs and the increase of amount of transmitted information further expected in the future demands the increase of signal transmission speed.
On the other hand, under a mobile communication environment, the deterioration in transmission characteristics is caused by multi-path fading due to the existence of multiple delay paths. Particularly, at the time of transmission of high-speed signals, since not only multi-path fading but also frequency selective fading increases in influence, a technology for overcoming this frequency selective fading is indispensable.
By the way, in an uplink (mobile station base station) of a mobile communication system represented by mobile phones and the like, since multiple mobile stations access the same base station at asynchronous timing, inter-user interference occurs. As technologies for preventing this interference, transmission methods such as DS-CDMA (Direct Sequence-Code Division Multiple Access) transmission, MC-CDMA (Multi-Carrier-Code Division Multiple Access) transmission and the like are used. These transmission methods reduce the inter-user interference while obtaining a frequency diversity effect by giving a specific spreading code to each user, spreading transmission signals in a wide band and performing communication.
However, since the above-mentioned transmission methods are spreading communications in a limited band and inter-user interference remains, the number of users being capable of performing communication at the same time is limited. On the other hand, it is possible also to use the communication giving a specific carrier frequency to each user without spreading (what is called frequency division multiple access: FDMA). However, this communication method has a fatal disadvantage that although no inter-user interference occurs, no frequency diversity effect is obtained and the above-mentioned frequency selective fading cannot be overcome.
Thereupon, there has been published a technique of spread spectra aiming at a frequency diversity effect but making users orthogonal to each other in frequency (non-patent literature 1). This technique repeats a transmission signal block consisting of Q data symbols at L times (that is, the spreading rate is L), gives a frequency offset specific to each user to a repetition sequence of it and performs transmission. Such transmission makes discrete spectra in which Q signal spectra in total appear in frequency points the number of which is L times the inverse number of the period of repetitions (namely, Q×L). As a result, since there are (L−1) sets of free frequency points, this technique arranges transmission signals of L users at maximum so as not to overlap each other (that is, so as to make them orthogonal to each other), performs transmission and enables a reception side to obtain a path diversity effect while preventing inter-user interference.
In relation to the above-mentioned technique at the transmission side, it is often performed that the reception side removes a frequency offset specific to each user from a received signal and then estimates a transmission data block by means of a least mean square error estimation or a matching filter detection. Recently there has been performed a study for applying this technique to the uplink of DS-CDMA (non-patent literature 2).
Non-patent literature 1: M. Schnell and I. de Broeck, and U. Sorger, “A promising new wideband multiple-access Scheme for future mobile communications systems”, European Transactions on Telecommunications, VOL. 10, No.4, July-August 1999.
Non-patent literature 2: Yosikazu Goto, Teruhiro Kawamura, Hiroyuki Atarasi, and Mamoru Sawahasi, “Uplink Variable Spreading Rate/Chip Repetition (VSCRF)-CDMA Broad Band Radio Access”, Technical Report of the Institute of Electronics, Information and Communications Engineers, RCS2003-67, pp.91-98, Yamagata, June 2003.